Evaporative emissions of fuel vapor from a vehicle having an internal combustion engine occur principally due to venting of the fuel tank of the vehicle. When the vehicle is parked, diurnal changes in temperature or pressure of the ambient atmosphere cause air to waft into and out of the fuel tank. Some of the fuel inevitably evaporates into the air within the tank and thus takes the form of a vapor. If the air emitted from the fuel tank were allowed to flow untreated into the atmosphere, it would inevitably carry with it this fuel vapor. The fuel vapor, however, is a pollutant. For that reason, federal and state governments have imposed increasingly strict regulations over the years governing how much fuel vapor may be emitted from the fuel system of a vehicle.
One approach that automobile manufacturers have long employed to reduce the amount of fuel vapor that a vehicle emits to the atmosphere involves the use of a storage canister. In this approach, a tube, often referred to as a "tank tube," is used to connect the air space in the fuel tank to the storage canister. Inside the storage canister is contained a sorbent material, typically activated carbon, whose properties enable it to adsorb the fuel vapor. Consequently, when air flows out of the tank, the tank tube carries it to the storage canister wherein the fuel vapor is adsorbed into the sorbent material There the fuel vapors are temporarily stored so that they can be burned later in the engine rather than being vented to the atmosphere when the engine is not operating.
FIGS. 1 and 2 illustrate one type of storage canister, generally designated 10, typically used in the automotive industry. FIG. 1 shows the canister in a perspective view, whereas FIG. 2 shows it in cross-section. The storage canister 10 comprises a container 18 that is partially divided by partition 24 into two compartments 20 and 22. An intercompartmental flow passage 26 connects these compartments.
The storage canister 10 has a tank port 12 and a purge port 14, both of which communicate with the first compartment 20. The tank port 12 connects to the tank tube 7, and thereby allows the air space in the fuel tank 8 to communicate with the first compartment 20. To the left of the tank port 12 as viewed from the perspective of FIG. 2, the purge port 14 connects to a purge line 19. Through a purge valve 15, the purge line 19 connects to the air intake passage 9 of the vehicle 11. (Air flowing into the air intake passage 9 is mixed with fuel, and the mixture eventually drawn into the cylinders for combustion.) The purge valve 15 is closed when the engine is not running. When the engine is running, however, purge valve 15 is opened in and thereby allows the storage canister 10 via the first compartment 20 to communicate with the air intake 9.
The storage canister 10 also features a vent port 16 that communicates with the second compartment 22. The vent port 16 connects to a vent line 6. The vent line 6 communicates with the ambient atmosphere through a vent valve 17. Typically controlled via a solenoid, the vent valve 17 is normally held open. When opened, the vent valve 17 allows the storage canister 10 via the second compartment 22, vent port 16 and vent line 6 to communicate with the atmosphere. The vent valve 17 is closed when the storage canister 10 is being tested for leaks.
Evaporative emission control systems of this type essentially have two phases of operation. During the storage phase when the engine is off, the system operates with the purge valve 15 closed and the vent valve 17 opened. When the pressure in the fuel tank 8 is high relative to atmospheric pressure, air from the tank and the fuel vapor it carries flows into tank tube 7 and through tank port 12 into storage canister 10. Inside the storage canister 10, the fuel vapor is adsorbed by the sorbent material 28 as the air that carried it flows not only through the first compartment 20 but also through the second compartment 22 via intercompartmental flow passage 26. Although a high percentage of the fuel vapor is adsorbed into the sorbent material 28, the air as it exits the canister 10 via vent port 16 carries with it some unadsorbed fuel vapor to atmosphere.
During the regenerative phase of operation when the engine 90 is running, the system operates with both the purge valve 15 and the vent valve 17 opened. A vacuum is developed within the intake manifold as a result of the combustion occurring within the cylinders of the engine 90. This vacuum ultimately causes fresh air from the atmosphere to be drawn through vent valve 17 and into the storage canister 10. Specifically, the air is pulled by vacuum through vent port 16, second compartment 22, flow passage 26, first compartment 20 and out purge port 14. Inside the storage canister 10, as the fresh air flows through the sorbent material 28, it strips it of the fuel vapor that it had adsorbed during the previous storage cycle. The sorbent material 28 is thus regenerated for the next storage phase. The purged fuel vapors are carried by the air stream through purge line 19, purge valve 15, air intake passage 9 and to the cylinders where they are consumed as fuel during combustion.
During the storage phase, the fuel vapors previously adsorbed by the sorbent material 28 may also return to the fuel tank 8 when the pressure in the tank lowers relative to atmospheric pressure. This happens when the temperature inside the fuel tank 8 drops and the fuel vapors condense. Being normally open, the vent valve 17 under such conditions allows air into the storage canister 10 and relieves any vacuum.
Due to the increasingly stringent air quality standards, the automotive industry has pondered several ways of further reducing the emissions of evaporated fuel. Thought has been given to increasing the size or number of compartments in the storage canister 10. Those approaches have been deemed undesirable due to excessive cost and bulk. Various proposals for heating the storage canister 10 electrically have also been considered. Those approaches have also proved undesirable due to the electrical power they would require.